This invention relates in general to methods and apparatus for transferring energy to a magnetically confined plasma. More particularly, this invention relates to a method and apparatus for heating a tokamak-confined plasma to thermonuclear temperatures by injecting an intense, pulsed, space-charge-neutralized ion beam into the plasma.
Various techniques of heating tokamak-confined plasma have been proposed in controlled thermonuclear fusion research in an effort to provide an ionized gaseous plasma of sufficient density and temperature to sustain fusion reactions. Heretofore such reactor conditions have not been attained because insufficient heating, plasma-confinement instabilities, and energy-loss mechanisms prevent the plasma from reaching the required temperatures.
It is generally agreed that ohmic heating by the main plasma current is ineffective near reactor temperatures because the plasma resistivity is a sharply decreasing function of temperature. Present-day experiments show that chemically heated tokamaks fall for short of reactor temperatures.
Since ohmic heating is insufficient, supplementary heating is required and techniques such as heating with neutral beams, microwave power and intense electron beams have been proposed. It is necessary that the power produced by these supplementary techniques be deposited near the center of the reactor plasma so that the energy is confined in the plasma and does not escape out of the plasma to the walls of the tokamak, thus introducing impurities from the wall into the system. These impurities, at best, cause inefficient heating--they may even result in the cooling of the confined plasma.
The injection of neutral beams into the confining magnetic field is recently regarded as the most promising method of supplementary heating. However, neutral beams can only be efficiently produced for energies less than 160 keV for deuterons (80 keV for protons). Considerably larger energies are needed if the neutral beam is to be deposited near the center of the reactor plasma. Microwave power can be delivered to the tokamak by waveguides attached to openings in the side walls, or by large coil structures inside the main vacuum chamber. This approach is limited by difficulty in controlling where in the plasma the microwave power is deposited, and also by anomalous scattering or anomalous absorption of the microwave power in the outer regions of the plasma due to parametric instabilities.
The injection of intense pulsed electron beams is a possible means of heating tokamak plasmas. However, the time required for relativistic electrons to deposit their energy into a plasma is very long so that some anomalous stopping of the beam must be invoked. This is very speculative and, even if there is an anomalous stopping, there may also be associated energy loss from the plasma. For instance, experiments have shown that discharges with runaway electrons can be very destructive to tokamak liners.
In ohmic heating, heating with neutral beams and heating with microwave power, the tokamak current is driven, i.e., the plasma is the secondary of a transformer whose flux runs through the center of the tokamak. This current-control system represents a very large part of the cost and complexity of a tokamak.